After reading both sides, I'd say it would certainly be one of the best matchings proposed for a while, with good arguments on both sides.
Some other things to think about:
+ whilst the Spartans were very fit and well trained, the Predators weren't slouches; they were (presumably) trained as, well, predators, to pass a coming-of-age test to the death. Theirs was also a warrior culture.
+ Spartan weapons were bronze: such a shield would be almost no defence against the Predators' weapons, which would tear it up like butter
+ for what limited advantage it would offer, as above, only the richer hoplites wore body armour
+ the Spartans' primary fighting weapon and technique was phalanx with doru (spear). Forget all the sword stuff in "300", most of their warfare was phalanx to phalanx. Yes, they did show it, with the overlapping shields to protect one-another. A Predator would just jump into the middle and massacre them from the inside. Oh, he'd be taken down, but not before killing half the phalanx. Their swords were only a secondary weapon if their spears broke or the phalanx broke up for some reason - again, they're bronze and would crumble against Predator-compound blades.